Aquatica is across the street from SeaWorld's exit. The walk would include the length of the parking lots and entrance/exit driveways. It's a moderate, but doable, walk.
When I was at SeaWorld, I saw a shuttle stop indicated on the park map. It was labeled as shuttle service to Aquatica, Discovery Cove, and Busch Gardens. I don't know how this works, so you should check with the park before you visit. It looks like there may be a continuous shuttle between SeaWorld and Aquatica.